1. Field of the Invention
The present invention relates to a driving circuit for a light-emitting diode (LED) lamp. More particularly, the present invention relates to a driving circuit for an LED lamp including fewer strings each through which more current flows.
2. Description of the Related Art
A liquid crystal display (LCD) such as LCD monitor gradually uses an LED lamp as a backlight source. The LED lamp includes plenty of LEDs and hence the LEDs are coupled in series and/or parallel. A driving circuit for the LED lamp usually uses a direct-current (DC) to DC boost is converter so that a specific-purpose integrated circuit (IC) is required to control the DC to DC boost converter.
FIG. 1 is a schematic diagram illustrating a conventional driving circuit for an LED lamp. Referring to FIG. 1, an LED lamp includes a plurality of strings 21-2n and each string 2i has an input terminal 21i and an output terminal 22i, where n is a positive integer and i is any integer from 1 to n. Each string 2i includes a plurality of LEDs coupled in series between the input terminal 21i and the output terminal 22i. A driving circuit for the LED lamp includes an alternating-current (AC) to DC converter 1 and a DC to DC boost converter 7. An AC voltage of 90-264 Vrms is input to the AC to DC converter 1. The AC to DC converter 1 converts the AC voltage into two DC voltages. A DC voltage of 12-19V is outputted from a terminal 121 to the DC to DC boost converter 7. Another DC voltage of 5V is outputted from a terminal 122 to a mainboard circuit 2.
The DC to DC boost converter 7 includes a boost circuit 71, an output filter circuit 72, a pulse-width modulation (PWM) controller 73 and an overvoltage detector 74. The boost circuit 71 converts the DC voltage of 12V-19V into a pulsating DC voltage. The output filter circuit 72 filters the pulsating DC voltage into a DC voltage to output to the input terminal 211-21n of the strings 21-2n to drive the LED lamp. The PWM controller 73 is a specific-purpose IC having a plurality of feedback terminals coupled to the output terminal 221-22n of the strings 21-2n, respectively, for controlling the current balance and short protection of the strings 21-2n; a dimming terminal coupled to a dimming control signal outputted from a terminal 123 of the mainboard circuit 2 for controlling the brightness of the LED lamp; and, an enable terminal coupled to an on-off control signal outputted from a terminal 124 of the mainboard circuit 2 for controlling whether the PWM controller 73 works or not. The PWM controller 73 outputs a PWM signal from a terminal 125 to control the amount of energy outputted from the boost circuit 71 so as to control the amount of current flowing through the LED lamp.
However, LED lamps fabricated by different manufacturers or even by the same manufacturer often differ in the number of strings, such as 1-8 strings. Accordingly, a PWM controller IC is usually designed to have more feedback terminals to be adapted for more LED lamps with different number of strings and each feedback terminal allows more current to flow thereto. The specific-purpose PWM controller IC is certainly expensive and hence the driving circuit using the specific-purpose PWM controller IC is also expensive. In addition, for example, a PWM controller IC (model name “TA9690GN” fabricated by O2Micro Inc.) has 8 feedback terminals and each feedback terminal allows current of about 50 mA to flow thereto. An LED lamp in a 21.5-inch LCD panel (model name “LM215WF4” fabricated by LG Corp.) includes 2 strings and each string allows current of about 160 mA to flow therethrough. If the LED lamp in the LCD panel (model name “LM215WF4”) is driven by the PWM controller IC (model name “TA9690GN”), it will waste design cost.